Aluminium-lithium alloys based on the aluminium-lithium-copper and aluminium-lithium-copper-magnesium systems have been developed to the stage where they are currently being considered for large-scale commercial use on the next generations of civil and military aircraft. The attractiveness of such alloys as replacements for established non lithium-containing aluminium alloy lies in their reduced density and increased stiffness but widespread application of these materials in aerospace structures will be dependent upon attainment of a satisfactory combination of many properties. The aluminium-lithium-copper-magnesium alloy registered internationally under the designation 8090 provides reduced density and increased stiffness in combination with strength, fracture toughness, corrosion resistance, fatigue resistance and ease of production at a level far in advance of the first aluminium-lithium alloys. Nevertheless there remains a perceived problem with regard to current aluminium-lithium alloys in regard to low fracture toughness in the short transverse direction. It might be that a low fracture toughness in the short transverse direction. It might be that a low fracture toughness in this axis presents no real barrier to use of the alloys in normal applications because the materials will not be subjected to usage which presents a stress on the axis but it remains something of a barrier to confidence in the new materials and might conceivably affect service life in some situations. The 8090 alloy for example, when aged to yield a tensile strength of 500 MPa or more which is typical of the modern high strength aerospace 7000 series alloys in the T76 condition, can exhibit low levels of fracture toughness in the short transverse direction typically 11 or 12 MPa (m).sup.1/2 as against 18 to 20 MP (m).sup.1/2 for the 7000 material whilst fracture toughness in other orientations of the 8090 alloy is more than acceptable.
The problem or perceived problem is not new-found and various tentative explanations have been advanced previously in the prior art. It is known that fracture in the short transverse plane (whether crack growth occurs in the longitudinal direction or the transverse direction orthagonal to the applied stress) occurs along grain boundaries and is brittle in nature showing little evidence of local ductility in those materials exhibiting low short transverse fracture toughness. The tentative explanations already made in the open literature embrace the following possibilities: localisation of the plastic strain at grain boundaries; grain boundary embrittlement by traces of hydrogen or low melting point metallic elements such as sodium, potassium or calcium; and the formation of large phases at the grain boundaries containing lithium, copper and possibly magnesium. This invention provides a convenient solution to the problem and studies made in relation to the invention indicate that these previously proposed explanations do not go the root of the matter though some of them relate to phenomena which will make some degree of contribution to the problem in certain circumstances.
Those present day aluminium-lithium alloys which are produced by the ingot metallurgy route rather than rapid solidification routes, are subject to the normal processing steps used and well established in the art for other species of precipitation hardening aluminium alloys, namely: casting to ingot; homogenisation heat treatment; forming to semi-finished product or product; solution heat treatment; quenching and artificial ageing at elevated temperature. In some alloys/tempers/products there is a cold working stage prior to the artificial ageing to secure an enhanced ageing response. The aim of the ageing treatment is to promote accelerated decomposition of the pre-existing supersaturated solid solution yielding the required strengthening precipitates.
Various artificial ageing treatments are known in the art in regard to aluminium-lithium alloys. Choice of ageing time and temperature permits ageing to peak strength, underageing, or overageing as required. Duplex ageing treatments are known, these being treatments in which the material is held at first one temperature (for the first stage of treatment) then held for a second period at a different temperature. As far as is known, those ageing treatments currently adopted for aluminium-lithium alloys aim to maintain the material in thermal equilibrium during each ageing period to promote a uniform precipitation of the strengthening phase or phases. We have found that the short transverse fracture toughness and ductility of aluminium-lithium alloys of the alunimium-lithium-copper-magnesium system can be significantly improved by imposition of an auxiliary heat treatment after ageing and our investigations of the phenomenon suggest that the auxiliary heat treatment will be effective also for other species of aluminium-lithium alloys such as alloys which contain copper but not magnesium and those alloys which contain zinc with or without copper and/or magnesium. Whilst the treatment might be expected to be of benefit to some degree in all alloy tempers it provides particularly a significant improvement in those product forms and tempers in which in the absence of such treatment the fracture mode would be a brittle intergrannular fracture.
Previously we had investigated the effects of a secondary ageing treatment upon 8090 plate material in the T8771 condition (that is material aged for 32 hours at 170.degree. C.) and based on secondary ageing times of 1 hour or more at temperatures of 170.degree. C. to 230.degree. C. it was concluded that some slight improvement in the short transverse fracture toughness of the material could be obtained by duplex ageing. A brief mention of this conclusion is given in a paper by C. J. Peel and D. S. McDarmaid given at pages 18 to 22 in the May. 1989 issue of Aerospace, which is the journal of the Royal Aeronautical Society. The best result that had been obtained by such a secondary ageing temperature was an improvement in short transverse fracture toughness as reflected by crack propagation in the longitudinal direction (hereinafter termed S-L fracture toughness) from approximately 20.5 MPa(m).sup.1/2 to 26 MPa(m).sup.1/2 following ageing for 1 hour at 210.degree. C. The practice used in this method is typical of that used in ageing practice in that the material was heated and cooled slowly to achieve thermal uniformity and held at temperature for an appreciable time in the expectation of securing an ageing response.
In contradistinction to our earlier result and expectation it has now been discovered that a more pronounced benefit in terms of improvement to short transverse properties can be achieved by use of a new heat treatment which is not intended to promote an ageing response and which is different in nature to those known in the art for the purposes of artificial ageing.